1. Field of the Invention
The present invention relates to an improved electrical contact for use in a current-interrupting unit and, more specifically, to an improved female electrical contact structure for a current-interrupting unit of a circuit interrupter. Even more specifically, the present invention relates to an improved female interrupting contact for a high-voltage current-interrupting unit usable in or as a circuit interrupter. Related inventions are described and claimed in commonly assigned U.S patent applications, Ser. No. 415,107, filed Sept. 7, 1982 in the name of Walter P. O'Leary, and Ser. No. 594,714, filed Mar. 29, 1984 in the names of Raymond P. O'Leary and Walter J. Hall, which is a continuation application of Ser. No. 415,761, filed Sept. 7, 1982 (now abandoned).
2. Brief Discussion of the Prior Art
Numerous types of circuit interrupters are well known. One type of circuit interrupter utilizes a disconnect switch, which includes a blade movable between closed and open positions relative to a stationary contact. Typically, the blade is pivotally mounted for opening and closing movement on a hinge which is carried by a first insulator, and the stationary contact is carried by a second insulator spaced from the first insulator. The hinge is electrically connectable to one side of a circuit or line and the stationary contact is electrically connectable to the other side of the circuit or line so that, with the blade closed, it engages the stationary contact and the circuit or line is continuous. When the blade is open, it disengages and separates from the stationary contact, and the circuit or line is discontinuous or interrupted.
When a circuit interrupter including such a switch is intended to render discontinuous an energized high-voltage circuit or line, it must typically have loadbreak capability. That is, upon opening of the switch, load current or other current flowing in the circuit or line must be broken. Loadbreak capability requires that the switch, or other circuit interrupter, have the ability to extinguish the high-voltage arc which necessarily forms when an energized, high-voltage circuit or line is opened. To this end, a circuit interrupter, such as a loadbreak switch, typically includes a current-interrupting unit. The current-interrupting unit may be stationarily mounted so that the blade swings relative thereto during its opening and closing movement. When the blade is closed, the interrupting unit may be either (a) connected in electrical parallel with the blade, in which case the majority of the current usually flows through the blade and the stationary contact because of the lower electrical resistance thereof, or (b) electrically discontinuous from the blade and the stationary contact so that it carries no current. In either event, as opening of the blade is initiated, facilities cause the current formerly flowing in the blade and the stationary contact to flow through the interrupting unit alone.
The interrupting unit includes interrupting contacts, one of which is (or may be made) electrically continuous with the stationary contact, while the other of which is (or may be made) electrically continuous with the blade. As the blade continues to open, the interrupting contacts separate, resulting in the formation of a high-voltage arc therebetween, which prevents or eliminates the formation of any such arc between the separating blade and stationary contact. As the blade moves well away from the stationary contact, the arc between the interrupting contacts, which are typically maintained in an enclosed, controlled environment, is extinguished by one of a variety of techniques.
One technique for extinguishing the high-voltage arc which forms between the interrupting contacts is to move the contacts relatively apart while simultaneously interposing between the separating contacts an elongated insulative member called a trailer, which conformably moves through a bore in an insulative cylinder, called a liner. In such a trailer-liner interrupting unit, the trailer usually moves with one of the interrupting contacts, which also moves through the bore in the liner, while the liner and the other interrupting contact remain stationary. The trailer and liner are usually both made of, or include, a so-called arc-extinguishing material, such as horn fiber, boric acid, or various plastics such as those sold under the trademarks Delrin and Lucite. The arc which forms between the separating interrupting contacts is elongated by their separation and is constricted by the conformal engagement of the trailer with the bore of the liner. Moreover, the heat of the arc interacts with the trailer and the liner to evolve cooling, turbulent, de-ionizing gases therefrom. Arc elongation and constriction, combined with the cooling, turbulent and de-ionizing effects of the evolved gases, extinguish the arc, as is well known. Extinguishment of the arc results in interruption of current flow. When arc extinguishment occurs within the interrupting unit, the blade is sufficiently far from the stationary contact so that the dielectric strength of the air gap therebetween prevents the formation of an arc therein to thereby prevent re-initiation of current flow.
In interrupting units of the type described above, one of the interrupting contacts is usually stationary, while the other interrupting contact is movable with respect thereto. The movable contact, which may be a male contact, carries the trailer and typically moves through or out of the stationary contact, which may be a female contact, and into the bore of the liner, which may be located adjacent to an exit end of the female contact. Thus, both the male contact and the trailer are drawn through the stationary contact and the bore of the liner.
Another type of circuit interrupter utilizes an interrupting unit similar to that described above, and may take the form of a portable high-voltage loadbreak tool, for example, such as that sold under the registered trademark "Loadbuster" by S&C Electric Company of Chicago, Ill., the assignee of the present invention. Basically, such a portable loadbreak tool comprises an interrupting unit which is mountable to an insulated "hot stick." The loadbreak tool permits the opening under load conditions of high-voltage devices, such as cutouts or disconnects, which, while similar to the above-described switch, do not themselves possess loadbreak properties (i.e., lack an interrupting unit). Facilities, such as hooks, rings, or bails, are included with the interrupting unit so that, upon manipulation of the "hot stick," both the stationary contact and the blade of the cutout or disconnect are mechanically engaged thereby. This engagement is such as to electrically connect one interrupting contact to the stationary contact of the cutout or disconnect and the other interrupting contact to the blade, so that the interrupting unit may be electrically paralleled with the cutout or disconnect. Subsequent movement of the "hot stick" simultaneously effects disengagement of the blade from its stationary contact and causes separation of the interrupting contacts within the interrupting unit. Thus, as the blade disengages the stationary contact, the current formerly flowing therethrough is "forced" to flow through the interrupting unit of the loadbreak tool, and current is ultimately interrupted along the lines described above. After the cutout is opened, the hooks, etc. are disengaged therefrom for storage or further use of the loadbreak tool.
Whether used in a loadbreak switch, a portable loadbreak tool, or other type of circuit interrupter, at some point during the opening of the circuit interrupter, the interrupting unit must carry the full current formerly flowing in the switch, cutout, or disconnect. This ability to carry the full current requires that the interrupting contacts be firmly and positively, mechanically and electrically engaged prior to their separation. In prior art interrupting units, such mechanical and electrical engagement has been achieved by configuring the female, stationary contact as a hollow cylinder which has formed integrally therewith, or formed in one end thereof, a plurality of contact fingers which define a continuation of the cylinder's bore. The contact fingers may be biased inwardly toward the axis of the bore by appropriate formation thereof during manufacturing or by encircling them with one or more garter springs. The contact fingers, which have open cuts or spaces therebetween, must be somewhat flexible so that during movement of the male contact therepast and out of the bore during opening of the interrupting unit, the male contact is slidingly, positively mechanically and electrically engaged by the fingers to ensure a low-resistance electrical path through the interrupting unit. The fingers must also be flexible in order to permit the male contact (and the trailer) to re-enter the bore of the female contact when the interrupting unit is closed.
The formation of the contact fingers is an expensive, time-consuming operation. The fingers may be formed by making an appropriate number of spaced, longitudinal cuts in one end of the hollow cylinder. Each cut must be accurately located and extended so that the fingers are all similarly flexible. The fingers may also be individually formed, which necessitates their accurate individual attachment to the hollow cylinder. Regardless of the manner in which the fingers are formed, each of them typically includes a conductive, refractory portion and a conductive, flexible portion. The refractory portion resists melting and the formation of asperities thereon by the arc, while the flexible portion permits sliding engagement of the fingers with the male contact. Each finger, therefore, requires attachment, as by brazing or welding, of the refractory portion to the flexible portion.
When prior art interrupting units are used over a period of time, the fingers sometimes break loose or fracture due to mechanical abuse, over-extended use during which one or more fingers become work-hardened, or damage caused by the heat of the arcs which form between the interrupting contacts as the interrupting unit opens. The breaking or fracturing of one or more fingers may jam the interrupting unit so that it is unable to open or close.
Moreover, the cuts or spaces between the fingers result in the presence of a number of surfaces on which the arc forming between the separating interrupting contacts may "root." Electrical arcs tend to "root," or preferentially terminate, on sharp or pointed surfaces such as those presented to the adjacent edges of the fingers. The "rooting" of the arc on a non-refractory portion of one or more of the fingers may lead to the above-described finger breakage and may also cause aperities to form on the wall of the bore of the female contact, that is, either on the wall of the bore in the cylinder or on the continuation of the bore defined by the fingers. Such asperities can prevent free movement of the male contact (and the trailer) into and out of the female contact. Accordingly, the operation of the interrupting unit may be compromised.
In view of the above, one object of the present invention is the provision of an improved electrical contact for a interrupting unit which is simple and inexpensive to manufacture and which obviates the shortcomings of the above-described prior art contact. A further object of the present invention is the provision of an improved interrupting unit utilizing the improved electrical contact of the present invention which, along with other structure, leads to improved circuit interrupters which are more reliably operable.
When the interrupting unit is used with a switch or similar device, it can interrupt current "too early." Specifically, if the unit extinguishes the arc when the blade is relatively close to the stationary contact, the voltage of the circuit or line in which the switch resides may be sufficient to initiate an arc therebetween to the detriment of the switch.
Accordingly, another object of the present invention is the provision of an improved contact for an interrupting unit in which the moment of current interruption by the unit may be selected so that an arc does not thereafter form in a switch or the like with which the unit is used.